Gregory the Dialogist

Behold, His servants are not stedfast, and in His Angels He found folly: How much more in them that dwell in houses of clay, whose foundation is in the dust, which shall be consumed as by the moth? Though the Angelical nature, by being fixed in contemplation of the Creator, remains unchangeable in its own state, yet hereby, that it is a created being, it admits in itself the variableness of change. Now to be changed is to go from one thing into another, and to be without stability in one's self. …

Morals on the Book of Job, Book V ↗

Thomas Aquinas

His second argument comes from a comparison to the angels. It is from the greater when he says, "Even those who serve him are not stable and in his angels he finds evil." This opinion is clear according to the Catholic faith. The Catholic faith holds as certain that all angels were created good. Some of them fell through their own fault from the state of righteousness; some however attained a greater glory. The fact that the angels fell from the state of righteousness seems astonishing for two reasons. …

Commentary on Job ↗